I would gently, but firmly redirect the baby, and also encourage the 4yo to watch out for it and move out of the way.
When my babies bit my while nursing or while I was holding them, I would say "no biting" or something like that and put them down. With the frustration, I would still do something like this, and also immediately talk about what you think the frustration is. Oh, you want that toy? Please ask nicely. I would start teaching your baby to say and sign "please" right away.
As far as the 4yo goes, I would definitely encourage him to share with the baby if he tries to get something from him -- I ask dd to get ds another one of what he has, or ask her if he can have a turn with it. I would talk to him about the communication issue and about how he needs to move away to keep his body safe. I would also encourage him to play with "special" toys at the table or somewhere where the baby can't get them.
Unfortunately, you are going to have to help mediate this. Try to keep an eye out for it and intercept the baby before he can chomp down. If you can get him at the moment before he bites and speak for him, model saying and signing polite words, it will help both of your boys learn to read each other and communicate better.
I think my ds was older than yours is now by a few months when I started teaching him that he couldn't take things from dd when she was using them. He needed physical involvement from me to learn to wait his turn. We still have some sharing problems at 20 months and 4 years, but they are both pretty good with each other now.